It has been reported that some lactic acid bacteria show prophylactic action and defensive action against various infectious diseases (Non-patent document 1). It has also been reported that these actions of lactic acid bacteria are based on activation of cell-mediated host immunity, promotion of IgA secretion from mucosae, such as those of intestinal tract and respiratory organs (Non-patent document 1), and so forth. For example, it has been reported that lactic acid bacteria belonging to Lactobacillus casei induce production of cytokines such as IL-12 (interleukin-12) and IFN-γ (interferon-γ) by immunocompetent cells of hosts to activate cell-mediated host immunity and thereby defend the hosts from infection by influenza virus, and so forth (Non-patent documents 2 to 4).
IL-12 and IFN-γ are cytokines having an action of inducing differentiation of naive helper T cells into type 1 helper T cells (Th1), an action of activating natural killer cells (NK cells), and an action of promoting phagocytosis of cells such as macrophages, and are involved in defensive actions against infections by viruses or bacteria, and antitumor effect of hosts. Therefore, in order to acquire high prophylactic action and defensive action of lactic acid bacteria against infectious diseases, it is important to use a lactic acid bacterium having a potent IL-12 production-inducing ability. As such lactic acid bacteria, there is known the Lactobacillus paracasei FERM BP-11313 strain, which shows high survivability under acidic conditions, and superior IL-12 production-inducing ability (Patent document 1, in this reference, this strain is also referred to as MCC1375 strain).
There has also been suggested involvement of cell walls of lactic acid bacteria in the induction of IL-12 production by lactic acid bacteria (Patent document 2), and it has been reported that if lactic acid bacteria are treated with a cell wall-digesting enzyme (N-acetyl muramidase), the IL-12 production-inducing ability is spoiled (Non-patent document 5). It has been also suggested that RNAs contained in lactic acid bacteria participate in the induction of IL-12 production by lactic acid bacteria, and it has been reported that if dead cells of lactic acid bacteria killed by heating are treated with RNase, the IL-12 production-inducing ability is markedly spoiled (Non-patent document 6). There have so far been also provided immunostimulants utilizing RNA of lactic acid bacteria itself (Patent documents 3 and 4).
Since human bodies contain cell wall-digesting enzymes such as lysozyme having the N-acetyl muramidase activity against bacteria, it is considered that lactic acid bacteria taken into the bodies are influenced by those enzymes. Further, RNases exist everywhere in the environment, and they are extremely stable against heat. Therefore, they easily contaminate products, and they are not inactivated by sterilization or the like, and may remain in the products. Moreover, since RNases also exist in human bodies, for example, in saliva and digestive juices, it is considered that lactic acid bacteria are also influenced by RNases after they are orally taken into the bodies.
Even if lactic acid bacteria inherently have high IL-12-inducing ability, they may not maintain and exhibit sufficient IL-12-inducing ability in living bodies, because they are influenced by the cell wall-digesting enzymes (N-acetyl muramidase) and RNases existing in saliva or digestive juices. It is considered that secretion amounts of these cell wall-digesting enzymes and RNases differ among individuals, and it is strongly considered that such difference possibly provides differences of the effect observed among individuals.